All planets are vast clumps of dust and gas packed together by gravity. If there's lots of material, a gas giant is the result. If there isn't much material, you get a rocky planet. At least, that's what we thought.

But now Seung-Hoon Cha and Sergei Nayakshin of the UK's University of Leicester have come up with a rather intriguing complication for the longstanding core accretion model. In this current view, planet formation is a bottom-up process. Dust and gas clump together into disks around their star, and then they start smashing together into bigger and bigger pieces until, eventually, the force of gravity is strong enough to pull all the material together into a planet.

However, the researchers propose a new, top-down model that they call tidal downsizing. In this new theory, giant gas disks form far away from their star, further out than where most of the currently known planets are found. Over time, these disks would start to clump together into gigantic planets as much as ten times as massive as Jupiter. During this clumping process, heavier dust particles would fall through the gas into the center of the forming planet, creating a rocky core.

But here's where tidal downsizing gets really interesting. The gas disk all around the newborn planet works to push the planet out of its orbit towards its star. Eventually, the planet gets close enough that the tremendous tidal force of the star starts ripping the gas right off the planet. Once all the gas is gone, only the rocky core is left, and it takes up a new orbit much closer to its star.

So suddenly, we go from a faraway gas giant to a rocky planet much closer to its star, potentially in the habitable zone. The researchers believe the resulting planets could be anywhere from zero to ten times the mass of Earth - and since our own planet obviously falls within that particular range, it's conceivable that we're living on the leftover husk of a devastated gas giant.

That's probably edging into more fanciful territory - though it's one hell of an intriguing thought - but this new model does provide a mechanism for planets to form in solar systems that aren't good fits for the core accretion model. Even better, tidal downsizing seems pretty much designed to generate potentially habitable planets. It seems that you just can't keep a good rocky planet down...even if it does have to start out existence as a gas giant.